1. Field of the Invention
The present invention relates to a control system for a motor vehicle, and more particularly to a control system for a motor vehicle having an internal combustion engine and a continuously variable transmission wherein the engine is operated along a predetermined operation line expressed in terms of at least two operating parameters for the vehicle.
2. Description of the Prior Art
Conventionally, it is a driver that controls a throttle position of the throttle valve of an internal combustion engine and a gear ratio of a transmission by depressing an accelerator pedal and manipulating a shift lever in view of an operation state of a motor vehicle so as to operate the motor vehicle for crusing or acceleration or deceleration or hill climbing. In this system, the driver has to adjust the position of the throttle valve and gear ratio to meet an ever-changing demand on the motor vehicle expressed by a changes in engine RPM. and in load. Thus, it is the conventional practice to design a control characteristic of the engine such that a shaft torque increases with increasing the opening degree of the throttle valve at any given engine RPM. Accordingly, the driver must cover all of the shaft torque range below a full throttle valve characteristic curve of the engine as shown in a graph of FIG. 1. (A family of dotted line curves in the Figure show isofuel consumption lines.)
The conventional system explained above has an inherent drawback that it is difficult to operate the engine in the most efficient operation state, viz., an operation line which may be expressed by the most efficient engine RPM. and a shaft torque for any given engine output.
As shown in FIG. 1, good engine thermal efficiency (fuel economy performance) is obtained when the engine is operated within an operation range wherein the engine RPM. is relatively low and the shaft torque is relatively high. The engine must be operated at or near the full throttle position to obtain good thermal efficiency. The reason is that if the opening degree of the throttle valve is small, the resistance to the induction increases, resulting in an increase in a pumping loss to cause a reduction in the thermal efficiency of the engine. With the conventional system, opening the throttle valve by a small degree from an operation state wherein the engine RPM. is low results in an increase in shaft torque. If, now, in order to increase the vehicle speed, the accelerator pedal is fully depressed to open the throttle valve to its fully opened position, engine RPM. no longer remains in the low engine RPM. range and increases to a value high enough to produce a shaft torque high enough to accelerate the vehicle. Therefore, a high shaft torque is not produced at low engine RPM., unless another operation parameter like a shift position is changed to a small gear ratio, such as by selecting a third gear ratio rather than a first gear ratio. However, it is actually very difficult to set an optimum combination of the operation parameters to meet a desired operating mode, and furthermore, since the gear ratio changes in a step manner whichever one of a manual transmission and an automatic transmission is used, the vehicle operates unsmoothly if the engine is controlled to operate along the optimum operation line. Besides, since it is the conventional practice to provide a power enrichment area near the full throttle opening at each engine RPM., viz., so-called "air fuel ratio enrichment range," in order for the engine to produce its power at each engine RPM. over all of the range from low engine RPM. to high engine RPM., the improvement of the fuel economy is limited.